1. Field of the Invention
The present invention relates to a toner for use in developing an electrostatic latent image. In addition, the present invention also relates to an image forming method and an image forming apparatus, which produce images using the toner.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes:    (1) charging a surface of a photoreceptor, which serves as an image bearing member, by causing an electric discharging (charging process);    (2) irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image on the surface of the photoreceptor (light irradiating process);    (3) developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor (developing process);    (4) transferring the toner image onto a receiving material (transferring process);    (5) fixing the toner image on the receiving material (fixing process); and    (6) cleaning the surface of the photoreceptor after transferring the toner image (cleaning process).
In the fixing process, a method in which a receiving material sheet bearing a toner image thereon is fed through the nip between a pair of rollers, at least one of which is a heat roller, so that the toner image is fixed on the receiving material sheet, is typically used.
In the fixing process, it is preferable in view of energy saving that toner images are well fixed at a low fixing temperature. Therefore, it is preferable to use a low-temperature fusible toner, which can fuse at a low fixing temperature. However, such a low-temperature fusible toner typically has poor preservation stability, and poor resistance to physical stresses caused by shaking, rubbing, agitating, etc. Namely, low temperature fixability of toner, and the preservation stability and stress resistance of the toner establish a trade-off relationship.
Recently, a strong need exists for an electrophotographic image forming apparatus, which has a small size and can produce images at a high speed. Therefore, a need exists for a toner which has a good combination of low temperature fixability, preservation stability and stress resistance. In attempting to fulfill the need, various techniques have been proposed. One of the techniques is to use a toner in which the inertia square radius of the binder resin components of the toner falls in a specific range.
For example, a published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2005-266789 corresponding to US2005186497A1 discloses a toner including a styrene resin having properties such that when the o-dichlorobenzene-soluble components of the resin is subjected to a viscosity analysis using a high temperature GPC-Ralls-viscometer, the absolute molecular weight Mw of the components is from 1.0×103 to 1.0×105, the ratio of components having an absolute molecular weight Mw of not greater than 10,000 is from 40 to 90% by weight, the ratio of components having an absolute molecular weight Mw of not less than 1,000,000 is less than 10% by weight, and the intrinsic viscosity of the components is from 0.10 to 0.80 ml/g. In addition, the average molecular size Rw (i.e., inertia radius R) of the styrene resin is preferably not greater than 1 nm. It is described therein that when the absolute molecular weight Mw and the ratios are lower than the specific ranges, the melt viscosity of the resin seriously decreases. In contrast, when the absolute molecular weight Mw and the ratios are higher than the specific ranges, the melt viscosity of the resin seriously increases. In addition, it is described therein that when the ratio of components having an absolute molecular weight Mw of not less than 1,000,000 is not less than 10% and the inertia radius is greater than 1 nm, the fixability of the toner deteriorates.
JP-A 2007-286562 corresponding to US2006240352A1 discloses a toner including, as a binder resin, a hybrid resin in which a polyester resin and a vinyl resin are chemically bonded, wherein the polyester unit is included in the resin in an amount of not less than 50% by weight. In addition, the resin includes tetrahydrofuran (THF)-insoluble components in an amount of from 3 to 50% by weight, and THF-soluble components (polyester resin components) obtained by hydrolyzing the THF-insoluble components have molecular weight distribution properties determined by gel permeation chromatography (GPC) such that a main peak is present in a range of from 50,000 to 500,000 and components having an absolute molecular weight Mw of not less than 1.0×107 have an inertia square radius of from 50 to 100 nm. Namely, the point of this application is that the size of resin components having an extremely high molecular weight is not too large (i.e., the molecules spread medially).
In addition, JP-As 2007-127920 and 2008-96624 have disclosed techniques such that the inertia square radius of the binder resin components of toner are specified.
JP-A 2007-148399 corresponding to US2007105033A1 discloses a toner having properties such that when the THF-soluble components obtained by dissolving the toner in tetrahydrofuran (THF) for 24 hours at 25° C. are subjected to a GPC-Ralls viscosity analysis, the inertia square radius (Rt) at the peak top of a main peak is from 1.0 to 3.8 nm, and the ratio (Rt/Rp) of the inertia square radius (Rt) to the inertia square radius (Rp) of linear polystyrene having the same peak molecular weight is not greater than 0.8. The point of the application is that the low molecular weight components of the resin preferably are preferably branched, and thereby the binder resin can well entwine with other toner constituents, resulting in improvement of fixing property of the toner.
Thus, various techniques have been proposed in attempting to improve the low temperature fixability of toner. However, the trade-off between the low temperature fixability, and mechanical strength and heat resistance (high temperature preservability) cannot be solved. In the cases where toner images are fixed at a relatively narrow nip while a relatively high pressure is applied thereto (such as roller fixing methods) or toner images are fixed at a relatively high speed using a belt (belt fixing methods), the toner is preferably melted in a short time. In addition, in order to prepare glossy images, the toner preferably has a low melt viscosity. However, the melt viscosity of toner is too low, the hot offset resistance of the toner deteriorates.
Until now, the following methods have been typically used for improving the low temperature fixability of toner.    (1) The glass transition temperature of the toner (binder resin) is decreased;    (2) The molecular weight of the binder resin is decreased; and/or    (3) A crystalline resin is used as a binder resin.
When the method (1) is used, the high temperature preservability of the toner deteriorates. When the method (2) is used, hot offset resistance of the toner deteriorates and image forming members (such as photoreceptors) are contaminated because the toner is deformed and fused by an external force. In this case, a toner film is formed on the image forming members and thereby abnormal images such as images having a rice-fish form are produced. When the method (3) is used, the kneaded toner constituent mixture is pulverized at interfaces between the crystalline resin and other components, resulting in occurrence of contamination of image forming members.
Thus, conventional toners could not solve the trade-off problem.
Because of these reasons, a need exists for a toner having a good combination of fixability, preservation stability and stress resistance.